two pipes a and b together can fill a cistern in 4 hours. had they been opened separately, then b would have taken 6 hours more than a to fill cistern. how much time will be taken by a to fill the cistern separately?

two pipes a and b together can fill a cistern in 4 hours. had they been opened separately, then b would have taken 6 hours more than a to fill cistern. how much time will be taken by a to fill the cistern separately?